Many boats are driven by internal combustion engines, which may be of the inboard or outboard type. Outboard engines are typically pivotally mounted on the stern of the boat allowing for steering without the need for a separate rudder, and enabling adjustment of the angle between the propeller drive and the hull for changes in boat trim. The pivotal mounting also allows boats fitted with such outboard engines to run up on to beaches with the propeller pivoted above the lower surface of the hull, preventing damage thereof.
Inboard engines are typically attached through a transmission or gearbox directly to the propeller shaft, and are necessarily mounted toward the center of the boat with a long propeller shaft to allow the propeller to minimize the drive angle between the propeller shaft and hull waterline. Such installations are necessarily bulky, and installation of the engine toward the center of the hull provides undesirable weight distribution characteristics in planing type hulls, which ideally have the heavy engine installed toward the rear of the boat.
To provide inboard engine installations with the advantages of outboard engine installations detailed above, inboard/outboard installations have been developed. These installations couple an inboard engine mounted within the hull of the boat to a drive assembly and propeller arrangement similar to that of an outboard engine installation, the engine essentially being connected through a `leg` to the propeller. The engine can thus be mounted toward the rear of the boat, and steering and trim control provided in a compact installation.
The mechanical system through which the power is transmitted usually consists of a drive-train of complex gearing, together with a number of joint systems so as to achieve steering and a measure of trim control. Such systems are mechanically complex and intrinsically inefficient. The power which may be transmitted through such devices is limited by the physical size of the devices, requiring the use of multiple engines and drives for high performance boats.